Do you have to download a special version of Blender to use experimental features?
No, you don't need a special version of Blender. They come with Blender. You can enable them in various places in Blender. 

For example, the experimental rendering features:



Another example - add-ons:





For your specific case, enable the experimental rendering features like you mentioned, then switch displacement type in the material settings (this will work with something attached to the displacement input of the material output node).





For setting up displacement, you will likely want to use a subsurf modifier with the "adaptive" setting enabled.



Here is the example .blend:



Note: The viewport doesn't properly reload yet, so you have to keep switching between solid and rendered view.